refactor(backend): has_backend() helper + docker/enumerate split + ActiveAgent rename

Addresses PR #78 review feedback:

- New `has_backend(name)` on the backend package + abstract
  `BottleBackend.is_available()` on each concrete subclass.
  Replaces inline `shutil.which("docker") is None` checks in
  docker/cleanup.py:178 and smolmachines/enumerate.py:73.
  Docker → `shutil.which("docker") is not None`; smolmachines
  → `smolvm.is_available()`. Cross-backend `enumerate_active_
  agents()` skips backends whose `is_available()` is False so a
  docker-only host doesn't fail when iterating past
  smolmachines (and vice versa).
- Move docker `enumerate_active` + parser helpers out of
  cleanup.py into a new `backend/docker/enumerate.py`, mirroring
  the smolmachines/enumerate.py layout. cleanup.py is now
  purely about prepare_cleanup / cleanup; the active-listing
  concern owns its own file.
- Drop the `ActiveAgent = ActiveBottle` alias in dashboard.py.
  The canonical name is `ActiveAgent` (the thing running inside
  a bottle is always called "agent" in this codebase; the bottle
  is the container). Renamed `enumerate_active_bottles` →
  `enumerate_active_agents` to match.

Tests:
- `test_backend_selection.TestEnumerateActiveAgents
  .test_skips_unavailable_backends` locks down the
  `is_available()`-gated iteration.
- New `TestHasBackend` covers `has_backend("docker")` consulting
  the backend's `is_available`, and unknown-name → False.
- Existing tests follow the rename; the docker-availability-
  side-effect test in `test_docker_enumerate_active` moves up
  to the cross-backend layer (where the gate lives now).

607 unit tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/cli/list.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 import argparse
 import sys
 
-from ..backend import enumerate_active_bottles
+from ..backend import enumerate_active_agents
 from ..manifest import Manifest
 from ._common import PROG, USER_CWD
 
@@ -23,7 +23,7 @@ def cmd_list(argv: list[str]) -> int:
 
     # `active` enumerates every backend (docker + smolmachines)
     # so smolmachines bottles aren't hidden behind the env var.
-    active = enumerate_active_bottles()
+    active = enumerate_active_agents()
     if not active:
         print("no active claude-bottle bottles", file=sys.stderr)
         return 0